Hydroprocessing can include processes which convert hydrocarbons in the presence of hydroprocessing catalyst and hydrogen to more valuable products. Hydrocracking is a hydroprocessing process in which hydrocarbons crack in the presence of hydrogen and hydrocracking catalyst to lower molecular weight hydrocarbons. Depending on the desired output, a hydrocracking unit may contain one or more fixed beds of the same or different catalyst. Hydroisomerization or dewaxing is a hydroprocessing process that increases the alkyl branching on a hydrocarbon backbone in the presence of hydrogen and hydroisomerization catalyst to increase cold flow properties of the hydrocarbon.
Diesel fuel streams must meet cold flow property specifications particularly for winter fuel use. One cold flow property is “pour point” which is the temperature at which a hydrocarbon stream becomes semi-solid and loses its flow characteristics. A high pour point is generally associated with a higher normal paraffin content or a normal paraffin content comprising higher carbon number. Another cold flow property is “cloud point” which is the temperature below which wax in the hydrocarbon stream begins to form a cloudy appearance. The “cold filter plugging point” of diesel fuel is the temperature at which the presence of solidified waxes clogs fuel filters and injectors in engines. The wax also can accumulate on cold surfaces such as on a pipeline or heat exchanger tube and form an emulsion with water.
When hydrocracking gas oil, cold flow property specifications for diesel product can limit the obtainable diesel yield by requiring a lower diesel cut point. Unconverted oil (UCO) is the material boiling above the diesel cut point. A portion of UCO can be recycled in a single-stage or two-stage hydrocracking process configuration as recycle oil or simply recovered as UCO in a once-through hydrocracking process configuration. It is desirable to decrease the product diesel cold flow property temperature values without reducing the diesel cut point to preserve more diesel yield. This can be accomplished by adding a hydroisomerization unit to decrease cold flow property temperature values without decreasing the cut point for the UCO.
There is a continuing need, therefore, for improved methods and apparatuses for hydrocracking and hydroisomerizing hydrocarbon streams.